1. Field of the Invention
The present invention relates to a charge pump circuit.
2. Description of the Related Art
Battery-driven electronic devices such as cellular phones, PDAs (Personal Digital Assistants), etc., mount a circuit component which requires higher voltage than the output voltage of a battery (battery voltage). In order to supply a suitable power supply voltage to such a circuit component, a booster circuit such as a charge pump circuit, switching regulator, or the like, is employed.
In a case in which a charge pump circuit is mounted on a battery-driven electronic device, in order to drive such a device for a long time, it is desirable that the charge pump circuit and the control circuit thereof operate with reduced power consumption. Examples of such techniques which have already been proposed include: a technique in which a comparator judges whether or not the output voltage of the charge pump circuit has reached a predetermined voltage, and the frequency generated by an oscillator is switched in a stepwise manner (Patent documents 1 and 2); and a technique in which the driving capacity of a buffer for driving the charge pump is switched according to the output voltage (Patent document 2).
[Patent Document 1]
Japanese Patent Application Laid Open No. 2000-173266
[Patent Document 2]
Japanese Patent Application Laid Open No. 2004-222349
[Patent Document 3]
Japanese Patent Application Laid Open No. H9-293376
[Patent Document 4]
Japanese Patent Application Laid Open No. 2003-217291
[Patent Document 5]
Japanese Patent Application Laid Open No. H11-220872
An arrangement in which the driving state of a charge pump circuit is switched using a comparator has a problem of increased fluctuation of the output voltage. An arrangement in which the number of comparators is increased in order to suppress the fluctuation of the output voltage has another problem of an increased circuit area.